Marie de Medici wore it mounted atop her crown for her coronation on 13 May 1610, the day before her husband, France's King Henry IV, was assassinated.

The Beau Sancy, which has rarely been shown to the public in recent decades, will go on a world tour from March and will be exhibited in Hong Kong, New York, Rome, Paris, London and Zurich before being sold in Geneva.

According to Sotheby's, when the last German Emperor and King of Prussia fled to exile in Holland in 1918, the crown jewels -- including the Beau Sancy -- remained at the Kaiser's palace in Berlin.

At the end of World War II, the collection was transferred to a bricked-up crypt in Bueckeburg, where it was later found by British troops. It was returned to the House of Prussia, which is now auctioning it.